A smoke alarm comprises a smoke detector and its associated electrical circuitry. A typical modern day detector measures the amount of impurity in the air by detecting changes in an ionization current induced by a radioactive source.
In one embodiment the source may be an .alpha.-emitter, located at a cathode, which ionizes the air to generate electrons collected at an anode. The anode is, of course, positively charged with respect to the cathode. A grid, called a plate, with a small hole through which current may pass, is located between the cathode and anode. Air surrounding the detector is permitted to enter freely into the space between the plate and the anode. The smoke detector operates as a voltage divider, the plate-cathode voltage decreasing accordingly as the ionization current decreases. The ionization current decreases as the amount of impurity, such as smoke, in the air in the anode-plate region increases.
Known smoke alarms are often made to operate in residential or commercial buildings from the utility electrical power supplies--typically 110-120 VAC at a nominal 60 Hz frequency. The alarms may be distributed throughout the buildings, each connected to an available outlet. The distributed alarms are then interconnected so that all the alarms are activated if any single one detects smoke. The alarms activate horns which may typically be ordinary bells, such as are useable as doorbells, each comprising a coil with a ferromagnetic core and a clapper.
The circuitry commonly associated with a detector is designed to respond to changes in the detector plate voltage when the detector anode is maintained at a substantially fixed positive voltage with respect to the cathode. Thus, such circuitry will necessarily include rectifier means for generating a substantially DC voltage to power the detector. Threshold detector circuitry responsive to the plate voltage of the smoke detector may then supply power to the horn when the plate voltage drops below a predetermined threshold. In existing detectors this is done by connecting one terminal of the horn to one terminal of the utility power supply through a current limiting resistor. The other terminal of the horn is connected to the other terminal of the power supply through a switch, which is usually an SCR. The threshold detector circuitry then closes the switch when the plate voltage drops below threshold. In this arrangement the horn is said to be connected in parallel with the rectifier means and the smoke detector.
There are several requirements that such smoke alarms must meet in order to be safe for use and otherwise commercially acceptable. The smoke alarms themselves must not, for example, be fire hazards. Consequently, they must be protected against voltage surges in the line supply which might, absent protection, burn out electrical components and cause sparking. Furthermore, the alarms should be dependable under normal voltage conditions. This goal may be met by minimizing the number of electrical components in the alarm circuitry. Also, a visual indicator, such as a light emitting diode, is often provided to show that the circuitry is operating normally. Commercial acceptability requires also that the smoke alarms be economical to purchase and use. The commercial goals are met by providing for a low initial cost--also a consequence of minimizing the number of circuit components--and minimal use of electrical power.
Line transients are very common in residential power supplies. Present line operated detectors employing SCRs as switches incorporate RC line snubbers to eliminate false triggering. The line snubbers undesirably add to the cost of the units and decrease circuit reliability because of the possibility of eventual capacitor degradation with resulting short circuits.
Current surge protection for present day line operated smoke alarms typically requires the use of a fuse or wire wound resistor. Either device allows a high surge current, which may be associated with a line transient or a short circuit, to reach and melt components in the detection device, possibly disconnecting power to the detector. The only indication of the blown fuse or open resistor will be extinguishing of the power-on indicator light. This indication may not be immediately noted if there are a large number of units in the building. Furthermore, large voltage surges may cause dangerous arcing in such alarm units.
An existing detector will typically activate its horn by applying a pulsating DC current to the horn coil. The DC component of the current produces a residual magnetization of the pole piece. The residual magnetization is undesirable because it reduces the inductance of the coil and consequently results in an increase in current flow during operation thereby increasing operating cost. Avoidance of the residual magnetization is presently achieved by establishing an alternate current path to provide a reset current during normal operation. The alternate current path requires the use of otherwise unnecessary electrical components.
Present systems suffer yet another disadvantage from the fact that they may operate unstably when the amount of impurities in the air is just at the threshold of the alarm condition. Under such conditions the impurities will generally not be uniformly distributed in the air and the detector may be exposed to different amounts of impurity at different moments. Thus, the smoke alarm will be in an alarm condition at one moment and a non-alarm condition at the next. It would be desirable to have a smoke alarm which changes its threshold once an alarm condition occurs so that a small decrease in smoke content will not put the detector back into a non-alarm condition. Such a change in threshold would effectively be a change of state of the smoke alarm circuitry, or hysteresis effect, stabilizing the alarm condition.